Telephone system



W. H. RYAN TELEPHONE SYSTEM Dec. 30, 1941.

2 Sheets-sheet 1 Filed May 23, 1940 mvENToR William H. Ryan BYgtIAZ/I/,lALd-yl M MMI ATTORNEYS.

DCC. 30, W H RYAN TELEPHONE SYSTEM 2 Sheets-Sheet 2 ...3Q uws@ wat @2&5

Filel May 25, 1940 Nm3@ 6N NNN www w w INVENTOR Wam H. I?

ratented Dec. 3o, 1941 2,268,424 TELEPHONE SYSTEM William H. Ryan,Cambridge, Mass., assigner to Associated Electric Laboratories, Inc.,Chicago, Ill., a corporation of Delaware Application May 23, 1940,Serial No. 336,710

` 15 claims ucl. 179-27) The present invention relates to telephonesystems, and more particularly to improvements in apparatus forproviding call intercepting service in an automatic telephone system.

In telephone systems of the automatic type it is frequently desirable toprovide facilities whereby, when one or more of the substationsassociated with a multiprty subscribers line of` the system is taken outof service or disconnected. calls intended for the disconnectedsubstation are automatically routed to an operators position so that thecalling subscribers may be informed that the called substation is nolonger in service. Call intercepting service of this character is alsodesirable in other instances where calls to substations actually inservice are to be intercepted and routed to an operators position.

It is an object of the present invention to provide improved callintercepting apparatus of the character indicated, which is of simpleand economical arrangement, is positive and reliable in its operation tointercept calls intended for predetermined lines or substations of thesystem and is so arranged that it cannot be falsely operated.

Itis another object of the invention to provide improved callintercepting apparatus of the character described, which is arranged tobe used in the interception of calls to predetermined substations servedby the multiparty lines of the system, is primarily controlled by theringing equipment common to the lines of the system, and requires aminimum of equipment individual to the lines which are served thereby.

It is a further and more specific object 'of the invention to provideimproved call intercepting apparatus of the character described, whichonly operates to intercept a call to one of theassociated lines afterringing voltage of the code or frequency designating a call to beintercepted has been applied to the line for a predetermined timeinterval defined by one complete operating cycle of the ringinginterrupter embodied in the associated ringing apparatus.

The invention is illustrated in its embodiment in an automatic telephonesystem which comrality of ringing code leads, together with cycli.

cally operating apparatus for impressing differently coded ringingvoltages upon the various ringing leads during each cycle of operationthereof, and for impressing a pickup vpulse upon the pickup lead at thebeginning of each code cycle. This interrupting equipment is arranged tocontrol a group of relays individually corresponding to the ringing'codes and each operative during each ringing interval of thecorrespond-A ing code. The automatic switching equipment of the systemcomprises directively controlled selector and connector switches, eachof the connector switches being equipped with a code selecting switchwhich is directively controllable to select the code of the ringingcurrent to be transmitted over a called one of the multipartysubscribers lines. Each connector switch is also equipped with a ringingrelay which operates in response to a pickup pulse applied to the pickuplead and functions to impress ringing voltage of the selected code uponthe selected line. After a call has been routed through this apparatusto a called line arranged for call interception, apparatus jointlycontrolled by the application of ringing voltage to the selected line,the operation of a predetermined one of the code relays, and theapplication of a second pickup pulse to the pickup lead, functions tointercept the call only in the event ringing voltage is impressed uponthe selected line concurrently with the operation of a predetermined oneof the code relays during the cycle of operation of the ringinginterrupter defined by the first and second pickup pulses applied to thepickup lead. In the event the application of ringing voltage vto theselected line is not concurrent with the operation of this particularcode relay, the call intercepting apparatus is positively prevented fromresponding to ring- 4|) prises a plurality of subscribers lines, severalof which are of the multiparty type, together with automatic switchingequipment for setting up connections between the lines. The systemfurther comprises ringing apparatus of the coded ringing type which iscommon to the lines of the system and is arranged to be used inselectively signaling the substations associated with the multipartylines of the system. More specifically, this apparatus comprises apickup lead and a pluing current impressed upon the selected line durlngfurther cycles of operation of the ringing interrupter.

Further features of the invention pertain to the particular arrangementof the circuit elements whereby the above and additional operatingfeatures are obtained.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. The invention, bothas to its organization and method of operation, together with furtherobjects and advantages thereof, will best be understood by reference tothe specification taken in connection with the accompanying drawings inwhich Figs. 1 and 2,

o5 when placed end to end in the order named, illustrate an automatictelephone system having embodied therein the features of the inventionbrieiiy outlined above.

Referring to the drawings, the system there illustrated comprises anexchange at which are terminated aplurality of subscribers lines oi thesingle party type, one of which is indicated at I0, and a plurality ofsubscribers lines of the multiparty type, one of which is indicated atII. As illustrated in the drawings, the line IU is arranged to serve thesubstation A and the line II is arranged to serve a plurality ofsubstations including the three substations B, C and D. In the systemillustrated, provisions are made for bridged and divided ringing. asindicated by the fact that the ringer. I2 provided at the substation Bis bridged between thev positive line conductor I Ia oi ther line andground in series with a condenser |5; the ringer I3 provided at thesubstation C is bridged between the negative line conductor IIb of theline and ground in series with a condenser I6; and the ringer I 4provided at the substation D is bridged across the conductors IIa andIIb of the line I in series with a condenser Il.

For the purpose of setting up connections between the various lines of.the system, there is provided, in the exchange terminating these lines,automatic switching equipment which comprises a pair of line switches |5and 20, individually associated .with the lines III and and havingaccess to a plurality of selector switches. one of which is indicated at25. This equipment further comprises a plurality of connector switches,one of which is indicated at as being accessible to the selector switch25 and as having access to the multiparty subscribers line Il. It willbe understood that a plurality of selector and connector switches,corresponding to the switches 25 and 30, respectively, are provided forserving the subscribers lines of the system, the number of switches ofeach type depending upon the trafiic which the exchange is required tohandie. 'I'he line switches of the system are, on the other hand,individual to the various subscribers lines of the system. Preferably,the line switches are of the well-known rotary type, although anydesired commercial form of this switch may be used. Similarly, theselector and connector switches of the system may be of any desiredcommercial arrangement, although preferably these switches are of thewell-known Strowger type. In order more clearly to explain the operenceof a biasing element, not shown. For the purpose of controlling therelease magnet |34, the switch is provided with a pair of off-normalcontacts which are arranged to Vbe operated into engagement when thewiper |32 is stepped from its normal position to a position inengagement with one of the contacts ot the contact set |3|. Although thetwo line switches I5 and 2li have been only schematically illustrated,it is pointed out in the interests of clarifying the followingdescription, that each ot these switches comprises line and cut-oi!relays, in addition to' the usual rotary trunk selecting switch. Thus,the line switch 2li is illustrated as comprising a line relay RI havingwindings normally bridged across the conductors oi' the line I'I throughthe restored contacts of an associated cut-oil relay, which is not shownbut is arranged to be energized over a. circuit including the privateconductor ||c of the associated line I.

In order to develop the differently coded ringing voltages which areutilized in selectively sigation of certain of the circuits to which thepresent invention pertains, a portion of the control equipment embodiedin the connector switch 30 has been shown in detail. More particularly,this.

equipment comprises a slow-to-operate ringing control relay Ri'IlU, aslow-acting ring cut-oi relay RHU, and a third relay R|20, which isprovided for the purpose of determining the side of a selected line overwhich ringing current is to be projected. The equipment further comwiperover the contacts of the contact set |3I,

and a release magnet |34 which is operative to release the wiper |32 sothat this wiper may be restored to its normal position under the infill.

naling the subscribers served by the multiparty subscribers lines,ringing apparatus is provided winch includes a ringing current generatorand a ringing code interrupter 55. These pieces of apparatus are commonto the entire exchange and may be of any desired type and construction.Accordingly, they have been only schematically illustrated in thedrawings. It is pointed out. however, that in accordance withconventional practice, operation of this apparatus is initiated inresponse to the application oi ground potential to the ringing apparatusstart conductor CIOS. After operation of the ringing apparatus is thusinitiated, the ringing interrupter operates in a cyclic manner toimpress diiierent- 1y coded ringing voltages upon the five ringingleads, C203 to C201, inclusive, during each cycle of operation thereof.More particularly, the

coded voltage pulses respectively impressed upon these leads arerepeated during each cycle oi operation of the interrupter. yAlso inaccordance with conventional practice, the ringing interrupter 55 isarranged to impress a pickup pulse of ground potential upon the pickuplead C208 at the beginning of each cycle of operation thereof, thepurpose of these pulses being to control the ringing relays of thevarious connector switches so that these relays will only be operated toimpress ringing voltage on the line selected by the associated switch atthe beginning of a ringing cycle. The transmission of mutilated ringingcodes over selected called lines is thus prevented.

For the purpose of intercepting calls intended for predetermined linesor substations of the system, there is provided call interceptingapparatus which comprises an intercepting operators position 45, and acall intercepting circuit III, the operators position 45 terminating oneend oi.' an intercepting trunk 45 by way of which the intercepting callsare routed to this position. Since the trunk circuit apparatusterminating the trunk 46 at the operators position l5 may be of anyconventional arrangement, this apparatus has been only schematicallyillustrated in the drawings. Briefly described, the call interceptingcircuit III comprises a plurality of code relays, two of which areindicated at R200 and R230', which are individual to the iive codesdeveloped through operation of the ringing interrupter 55. Each of thesecode relays is arranged to be energized in response to the applicationof ringing voltage to the associated ringing lead. As indicated by thecircuit connections illustrated, the relays R200 and R200 are individualto the codes 1 and 5,

these two relays being respectively energized in h lays which areindividual to the lines arranged,

for call. intercepting service. Each of these relays. corresponds to aparticular code, the relays R2I0 and R2I0 being provided in conjunctionwith the multiparty subscribers line II and individually correspondingto the codes 1 and 5, respectively. The individual line equipment of theintercepting circuit further includes from one to three lalternatingcurrent relays in association with each line arranged for callintercepting service, the number of relays in each instance beingdetermined by the character of the ringing utilized in signaling thesubstations for which call intercepting service is provided. Thus thereis provided a relay R230 in association with the line II which isbridged between the negative line conductor IIb and ground in serieswith a condenser 234, so that it will respond to ringing currentprojected over the negative side of the line when divided ringing isutilized in signaling the substation C, for example. If the circuit isto be arranged for the interception of calls intended for the substationB, for example, a second relay R230', corresponding to the relay R230and bridged between the positive line conductor IIa and ground, isrequired. Further, if Vcalls intended for a substation arranged forbridged ringing and associated with the line II are -to be interrupted,a third relay, bridged between the conductors of the line II in serieswith a condenser, is required. The intercepting circuit 40 also includesa line switching relay R240 associated with the line II, similar relaysbeing provided which are individually associated with the other linesarranged for call intercepting service. AFor the purpose of selectingthe lines occupied with calls which should be intercepted, a finderswitch 210 is provided. This switch comprises a switching mechanism ofthe wellknown rotary type, a sloW-to-operate start relay R250, and acombined line test and line switching relay R260. More specifically, therotary finder switch comprises three sets of bank contacts, 21|, 212 and213, wipers 214, 215 and 216 individually associated with the enumeratedcontact sets, and means comprising a rotary magnet 211 for` driving thewipers over the contacts of their respective associated contact sets.

In considering the operation of the system it may be assumed, forexample, that a call intended for the substation B associated with theline II, is initiated at the telephone station A. When the call isinitiated a loop circuit is completed which includes the two conductorsof the line I and extends to the line switchl5. The completion of thisloop circuit results in operation of the line switch I5 to select atrunk line leading to an idle one of the selector switches, and themarking of the calling line I0 as busy in the bank contacts of theconnector switches having access thereto. Assuming that theselectorfswitch 25 is seized by the line switch I5, certain of thecontrol relays embodied in this selector switch operate to condition theswitch to respond tothe first series of impulses dialed at the callingsubstation. These relays, in operating, also function to complete acircuit for transmitting the usual dial tone signal over the establishedcalling loop circuit for the purpose of informing the calling subscriberthat the dialing operation may be started.

When the first digit of the directory number designating the calledsubstation B is dialed at the calling substation, the switch 25 elevatesits wipers to a position opposite the level of bank contacts terminatingthe trunks leadingito the connector switches having access tothe.desired line I I. Following this operation and during theinter-digit pause between the first and second digits, the wipers of theswitch 25 are automatically stepped over the contacts of the selectedlevel until a trunk leading to an idle connector switch of the selectedgroup is found. Assuming. for example, that the connector switch 30 isthe first available idle connector switch in the selected group, whenthe wipers of the selector switch 25 are positioned on the contactsterminating the trunk line extending to this connector switch, thesubscribers loop circuit is extended to` the switch 30 and operation ofthe selector switch is arrested in the usual manner. When lthe callingloop circuit is cut through to the connector switch 30 in the mannerjust explained, certain of the relays embodied in the selector switch 25operate to interrupt the dial tone signal circuit, and certain of thecontrol' relays embodied in the connector switch 30 operate to completea circuit extending by way of the private conductor of the trunkincoming to this switch for holding the preceding switches 25 and I5operated. More specifically, the private or release conductor of thetrunk incoming 'to the connector switch 30 is grounded in response tooperation of the line and hold relays embodied in this switch when thecalling loop circuit is extended to the switch. These relays alsofunction to complete a path for impressing ground potential upon theringing apparatus start lead CI09, thereby to initiate operation of thegenerator 50 and the ringing interrupter 55 in the event this apparatusis not already in operation. Following the operations just described,the connector switch 30 is conditioned to respond to the three remainingdigits of the directory number designating the desired substation B. Inresponse to the dialing of the second digit at the calling substation A,the wipers of 'the connector switch 30 are stepped vertically to apositionopposite the level of bank contacts at which are terminated thelines of the ten-line subgroup including the called line II. Followingthis digit, and during the inter-digit pause between the second andthird digits, the connector switch 30 is conditioned to rotate itswipersstep by step in accordance with the impulses of the third digit of thedirectory number designatingthe substation B. When this third digit isdialed, the wipers of the switch 30 are operated into engagement withthe set of bank contacts terminating the conductors of the line I I.During the inter-digit pause between the third and fourth digits,circuits are prepared for energizing the operating magnet |33 of theringing code selecting switch |30, and for determining the position ofthe relay RI20, all in accordance with the impulses of the fourth andnaldigit. The arrangement of the circuits forfcontrolling the switchingrelay RI20 may be such that this relay is energized in response to thedialing of a code-selecting digit having an odd number of'impulses such,for example, as one, three or ve impulses, and is not energized inresponse to the dialing of a code-selecting digit having an even numberof impulses. With such an arrangement, if a code-selecting digitcomprising one impulse is dialed at the calling substation, ythe relayRIZI will be energized at the end of the digit, and ringing voltagehaving the code 1 will be selected through operation of the switch I 30so that ringing voltage ofthe code 1 will be impressed upon the negativeline conductor II b. In the case under consideration,

however, wherein the ringer I2 provided at the substation B is bridgedbetween the positive line conductor Ila and ground, ringing current mustbe transmitted over this positive line conductor in order to energizethe ringer I2 and thus signal the substation B. Accordingly, thecode-selecting digit designating this substation will,

with the arrangement described, comprise an even number of impulses.Assuming, for example, that the code-selecting ,digit designating thesubstation B is the digit 4, the wiper |32 will be operated'to engageits associatedfourth contact during the dialing of this digit and willbe left standing in engagement with the second of the two contactsterminating the ringing lead C204 at the end of the digit. Morespecifically, the -impulses of the code-selecting digit are transmittedto the operating magnet |33 of the switch |30 so that the wiper |32 isoperated one step in response to each impulse of the digit.V

When this wiper is moved oiI normal, the oinormal contacts |35 areoperated -lnto engagement to prepare a circuit for energizing therelease magnet |34. Also, the relay RI20 is, in the case assumed, notenergized and remains in its restored position at the end of thecode-selecting digit.

In accordance with conventional practice,

suitable provisions are made in the connector switch 30 for testing aselected line, in this case the line II, for the purpose of determiningthe idle or busy condition thereof. In the event the line tests busy,the usual busy tone signal is returned over the established connectionto inform the calling subscriber that the desired connection cannotimmediately be obtained. On the other hand, if the called line testsidle, ringing current of the selected code is transmitted over theselected side o1' the line for the purpose of energizing the ringerprovided at the called substation. More specically, if the called lineis idle, certain of the relays embodied in the connector switch 30operate to prepare a circuit for energizing the slow-to-operate ringingcontrol relay RI00. 'I'his circuit is thereafter completed at thebeginning of the next cycle of operation of the ringing interrupter 55when ground potential is impressed upon the pickup lead C208, andextends by way of the grounded lead C208, the upper winding of RI00, theprivate Wiper of the switch 30, and through` the Winding of the cut-offrelay embodied in the line switch 23 to battery. When energized overthis circuit, the relay RI00 operates to close its preliminary makecontacts |03, thereby to complete a pref pared circuit for energizingits lower winding. Thereafter, the relay RI00 completes its operationfurther to prepare a talking circuit between the calling and calledsubstations, and to complete a path for transmitting ringing current ofswitch 30, thereby directly to energize the cut.

ofi.' relay embodied in the line. switch 2l. This relay now operates todisconnect the windings of the line relay RI from across the conductorsof the line II, thereby to clear this line of the shunt impedancerepresented by the line relay windings. With the relay Rill operated,ringing current is caused to traverse the windings of the ringer I2provided at the substation B over a circuit which extends by way of theselected ringing lead C203, the wiper |32, the contacts |2I, II| andIII, the positive line wiper of the connector switch 3|, the conductorIIa, the condenser I5, and the windings of the ringer I2 to inaccordance with the No. 2 code and accordingly the subscriber at thesubstation B is informed that a call intended for his substation hasbeen routed to the line II and should be answered. Portions of theringing current transmitted over the crcuit traced above are obviousLvshunted through the ringers respectively provided at the othersubstations associated with the line Il which are bridged between thepositive line conductor IIa and ground. The coded character of theringing current, as reproduced by these ringers, indicates to the othersubstation users, however, that the call is intended for the substationB. A portion of the ringing current is also transmitted over the callingend of the connection in the usual manner. thereby to signal the callingsubscriber that the called substation is being signaled.

The desired conversational circuit between the calling and calledsubstations is fully completed when the call is answered at the calledsubstation B. More particularly when the receiver at this substation isremoved from its supporting hook or cradle, a direct current conductivebridge is conducted across the conductors Ila and l Ib of the line I Ito complete a circuit for energizing the upper winding of the ringcut-off relay RI I0. This circuit may be traced as extending from groundby way of the ringing lead C2, the wiper |32, the contacts I2I, III andIBI, the conductor Ila, the bridge across thev conductors of the lineII, the conductor IIb, the contacts |02, H3 and |23, and the upperwinding of the relay RI|0 to battery. When its upper winding isenergized over this circuit, the relay RI I0 operates to close itspreliminary make contacts H5, thereby to complete a prepared circuit forenergizing its lower winding. Thereafter, the relay RI I0 at itscontacts I I I and I I3, opens two points in its own operating circuit,and two points in the above traced circuit for transmitting ringingcurernt over the line II At its contacts I I2 and I4, the relay RI I0completes the desired conversational circuit between the callingsubstation A and the called substation y B. When' the relay RI I0operates, it also funcof the code-selecting digit dialed at the callingsubstation. For example, if the substation C ls being called and thecode designating this substation is the code 4, the wiper |32 of thecodeselecting switch |30 will be left standing in engagement with itsassociated seventh contact at the end of the code-selecting digit, andthe re- 1ayR|20 will be operated. In such case, the path traversed bythe ringing current extends by way of the ringing lead C206, the wiper|32, the contacts |22, ||3 and |02, the lineconductor Hb, the condenser|6, and the windings of the ringer |3 to the grounded terminal of theringing current generator 50. Further, when the call is an swered at thecalled substation C, the upper winding of the ring cut-off relay Rl I0is energized over a circuit extending by way of the grounded ringinglead C206, the wiper |32, the contacts |22, ||3 and |02, the conductorIIb, the bridge across the conductors of the line the conductor lia, thecontacts |0|, and |24, and the upper winding of Rl l0 to battery. Thus,it will be seen that the ring cut-off relay R| I0 is energizedregardless of whether the relay Rl is energized or deenergized at thetime the call is answered. In the event the desired substation such, forexample, as the substation D, is arranged for bridged ringing, the codedesignating this substation may be selected by dialing either the odd orthe even-numbered digit designating the code corresponding to thissubstation. In such case, the return path for the ringing currentextends by way of the upper winding of the ring y cut-ofi relay Rl I0and the exchange battery to the grounded terminal of the ringing currentsource. Thus, it will be seen that provisionsare made for both dividedand bridged ringing, and that the ring cut-off relay Rl |0 iscaused tooperate in response to the answering of a call routed through the switch30 regardless of the type of ringing utilized or theside of the selectedline over which ringing current is projected.

Preferably, the control equipment of the connector switch is so arrangedthat the release of the operated switch train is entirely under thecontrol of the calling party at the substation A. With such anarrangement, when the connection is cleared out at the [callingsubstation, the loop circuit extending through the switches |5 and 25 tothe connector switch 30 is interrupted to cause the release of the lineand hold relays embodied in this switch, in the usual manner. When theserelays restore, holding ground potential is disconnected from theprivate or release conductor of the trunk incoming to the connectorswitch 30 so that the line and selector switches l5 and 25,respectively, are released. When these relays restore, the controlapparatus embodied in the connector switch 30 is restored to normal orin a manner well understood in the art. During the release of thecontro-l apparatus embodied in the connector switch 30, the previouslydescribed holding circuits for the relays R||0 and R|00, and theoperating circuit for the relay R|20, are interrupted causing theserelays to restore. Also, ground potential is removed from the privateconductor ||c of the called line whereby the cutoff relay embodied inthe line switch 20 is released to reconnect the windings of the linerelay R| across the conductors of this line. When ground potential isremoved from the private conductor of the line battery potential isimpressed upon this conductor through the winding of the cutoff relayembodied in the line switch 20, thereby to mark the line as idle in thebank contacts of the connector switches having access thereto. When theline and hold relays embodied .in the connector switch 30 restore, acircuit is also completed forenergizing the release magnet of theStrowger switching mechanism embodied in this switch, thereby to causethe wiper carriage structure of this mechanism to be restored to rotaryand vertical normal. Similarly, the prepared operating circuit for therelease magnet |34 is completed, whereby the wiper |32 is restored toits normal position. When this wiper is restored to normal, thelofi-normal springs |35 are disengaged to open the circuit forenergizing the magnet |34. Thus, the apparatus involved in setting upthe connection is fully released.

Referring now more particularly to the operation of the callintercepting circuit 40 to intercept a call routed by way of theconnector switch 30 to the line ll, it may be assumed, for example, thata substation normally designated by the code 1, and including a ringerbridged between the negative line conductor ||b and ground, has beendisconnected from service. In order to intercept calls intended for thissubstation, the relays R2|0, R230 and R240 are provided, which relaysare connected in the manner illustrated, and the conductors of the lineare connected through the normally open contacts 24| and 242 of therelay R240 to an available set of blank contacts in the nder switch 210.Further, the op` erating circuit for the lockout relay R2|0 is connectedto be controlled by the No. 1 code slave relay R220. y

With the circuit arranged in this manner, if ringing current of the codel is projected over the negative side of the line in the manner justexplained, a portion of the current is caused to traverse the condenser234 and the winding of the relay R230. The relay R230 will thus operateat the beginning of the rst ringing interval which follows the operationof the ringing relay R|00 embodied in the connector switch 30. Inoperating, the relay R230 completes, at its contacts 23|, a primaryoperating circuit for the lower winding of the line switching relayR240, this circuit extending by way of the grounded private conductorllc, the contacts 2|2 and 23|, and the lower winding of R240 to battery.When its lower winding is energized over this circuit, the relay R240partially operates to close its X contacts 243, 245, 249 and 249 only.At its contacts 245, the relay R240 completes an obvious circuit formaintaining its lower winding energized independently of the relay R230.At its contacts 243, the relay R240 prepares a circuit =for energizingits upper winding. At its contacts 249, the relay R240 prepares twoalternative operating circuits for the lockout relay R2 I0. At itscontacts 249', the relay R240 similarly prepares two alternativeoperating circuits for the lockout relay R2|0. If calls intended forsubstations associated with the line and designated by one or more ofthe remaining three codes are to be intercepted, the relay R240, inpartially operating, also prepares a pair of operating circuits for eachof the additional lockout relays individually corresponding to the othercodes. In this regard it will be understood that the circuits providedfor any required additional lockout relays are identical with thoseprovided in association with.

the two relays R2|0 and R2|0.

During each period when ringing current is passed through the winding ofthe relay R230, a portion of the current is transmitted by way of thelead C203 andk the condenser 202 to energize the winding of the No. 1code relay R200. This code relay, in operating, completes at itscontacts 20|, an obvious circuit for energizing the associated slaverelay R220. Thus, the control device or slave relay R220 and the relayR230 are concurrently operated during each 'ringing period of theselected code No. l. The relay R220. in operating, opens, at itscontacts l22|, one of the prepared operating circuits for the relay R2|0 and prepares, at its contacts 222, a second operating circuit for therelay R2| 0. Concurrently with these operations, the relay R200 opens,at its contacts 233, the second operating circuit for the Vrelay R2|0,and prepares, at its contacts 232, the ilrst mentioned operating cir-Vcuit therefor. Thus, the operating circuit for the relay R2|0 is heldopen due to the concurrent operation of the two relays R220 and R200.

The concurrent operation of the relays R220 and R230 to maintain theoperating circuit for the relay R2I0 interrupted, continues throughoutthe cycle of operation of the ringing interrupter 55 which followsoperation of the ringing relay RI00 embodied in the operated connectorswitch 30. In this regard it will be noted that the relay R2| 0 is ofthe slow-to-operate type and, accordingly. will not operate if one ofthe operating circuits therefor V`is inadvertently closed for short timeintervals occasioned by a difference in the operating periods of the'relays R220 and R230.

During this rst cycle of operation of the ringing interrupter 55, thelockout relays associated with the Aline II, and individual to the codesother than the code l, are all caused to operate. Thus, in the caseunder consideration, ringing current is not projected over the positiveside of the line and accordingly the relay R250' isV not energized.When, therefore, the code 5 relay R200' first operates to complete, atits contacts 20|', the operating circuit for the relay R220', the latterrelay functions to complete one of the prepared operating circuits forthe code 5 lockout relay R2|0'. This operating circuit extends fromground by way of` the contacts 248', 249 200' and 222', and the windingof the relay R2|0 to battery. 'I'he ringing interval during which therelay R2|0' is Venergized, over this circuit is suiliciently long topermit the operation of this relay. Upon operating, the relay R2|0'completes a locking circuit for itself, this circuit extending by way ofthe grounded private conductor llc, the contacts 2|I', and the windingof R2|0' to battery. After this circuit is completed, the relay R2|0opens one of the multiple circuits for energizing the lower winding of.the line switching relay R240 at the con- 1 tacts 2|2'. Thus, thelockout relay R2I0' corresponding to the code 5 is locked up. for theduration of the call. In a similar manner, the other lockout relaysindividually corresponding to the codes 2, 3, and 4, if provided inassociation with the line are locked up for the duration of the call. Inthis regard it will be apparent that if any one of the lockout relays iscontrolled by the relay R200, which is being energized by ringingcurrent of the code 1 transmitted over the line this latter relay willbe nonccncurrently operated or restored at a time when the code vrel-aycorresponding to the lockout relay is not operated or restored.Accordngly, the lockout relay controlled by the relay R230 will operateand lock to the grounded private conductor llc. Thus, all of the lockoutrelays with the exception of the relay R2|0, which 75 per winding ofR240 to battery.

lthis interrupter, a second ground pulse is impressed upon the pickuplead C200, thereby to energize the upper winding of the relay R240.Thecircuit for energizing this winding extends by way of the groundedpickup lead C208, the contacts 243 and 244', and the upper winding ofR240 to battery. When its upper winding is energized over this secondaryoperating circuit the relay R240 completes its operation, and in sodoing closes a locking circuit -for itself which extends by way of thegrounded private conductor I Ic, the contacts 2|2 and 244, and the up-After this circuit is completed. the relay R240 disconnects the pickuplead C200 from its upper winding terminal Vwhich the test lelay R250 isoperated when the wipers of the iinder switch 210 have been pomtioned onthe contacts terminating the conductors of the line At its contacts240,-the relay R240 opens a point in each of the two circuits forenergizlng the relay R2I0, whereby the lastmentioned relay is preventedfrom operating during succeeding cycles `of operation of the ringinginterrupter 55. At its contacts 240', the relay R240 opens a point inthe two operating circuits for the code 5 lockout relay R2I0'. In asimilar manner, the operating circuits for the other lockout relaysprovided in conjunction with the line are'opened in response to theoperation of the line switching relay R240. At its contacts 241, therelay R240 completes an 0bvious Vcircuit for energizing theslow-to-operate finder start relay R250. When thus energized the relayR250 operates further to prepare, at its contacts 25|, the operatingcircuit for the test relay R200. At its contacts 252 the relay R250completes a circuit for energizing the operating magnet 211 of theilnder switch. This circuit extends from ground by way of the contacts200, 252 and 210, and the winding of the magnet 211 to battery. Due tothe circuit interrupting action of the magnet controlled contacts 210,this magnet operates buzzer fashion and in cooperation'with itsassociatedv ratchet and pawl mechanism operates to drive the wipers 214to 210, inclusive, over the contacts of their` respective associatedcontact sets until the line is found. More specifically, when the wiper210 engages the test contact corresponding to the line I, theabove-mentioned circuit for enoperation of the finder switch isarrested. At`

its contacts 20| and 202, the relay R200 connects the conductors of theline to the con- Following the operation of the relay R260, and

during continued operation of the ringing interrupter 55, coded ringingcurrent is transmitted over the line II, through the operated contacts24| and 242, the wipers 214 and 215, the operated contacts 26| and 262,and over the conductors of the trunk 46 to the trunk circuit terminatingthe distant end of this trunk. This ringing current as thus transmittedto the operator's po'sition 45, may be utilized to control a signaldevice in the form of a drop or signal lamp, whereby the operatorattending this position is informed that a call is present on the trunk46 which should be answered. When the call is answered at this operatorsposition, a direct current conductive bridge is connected across theconductors of the trunk 46, thereby to complete one of the previouslytraced operating circuits for the ring Y attending this position mayconverse with the calling subscriber and dispose of the call in theusual manner by informing the subscriber that the called substation hasbeen taken out of service or that the desired party may be reached bydialing another number. intercepting circuit 40 is entirely under thecontrol of the calling subscriber. More specically, when the controlequipment of the connector switch operates in the manner described aboveto disconnect the private conductor Ilc of the line II from ground, allof the operated relays, namely the relays R240, R250, R260 and theoperated lockout relays R2I0, etc., are deenergized and restore. Whenthese relays restore, the call intercepting circuit is fully releasedand is prepared for further operation. In this regard it is pointed outthat the finder switch 210 is of the non-homing type, i. e., the wipersthereof are left standing in engagement with the contacts correspondingto the line I I until further operation thereof is initiated in responseto the rouiting of another call to the intercepting circui Y In theevent the aboveetraced call routed to the line II is not intended for asubstation arranged for call interception, the call intercepting circuit40 will be prevented from operating regardless of the side of the lineover which ringing current is projected during the ringing period. Insuch case the called substation will necessarily be designated by aringing code other than the code I. Accordingly, if the ringing currentis transmitted over the negative line conductor IIb to energize theringer provided at a called substation and incidental thereto the relayR230, this relay will be nonconcurrently operated with the No. 1 coderelay R200 during one or more intervals of the first cycle of operationof the ringing interrupter which follows the operation of the ringingrelay RI00. When the relay R230 iirst operates in response to ringingcurrent transmitted over the line Il, it functions to complete thepreviously traced circuit for energizing the lower winding of the relayR240. The

relay R240 is thus caused partially to operate to prepare the respectiveoperating circuits for the lockout relays R2I0, R2I0', etc. When,thereafter, the -relay R230 is operated during an interval when therelay R220 is restored, or vice The release of the call I versa, one ofthe two operating circuits for the relay R2I0 is completed. This circuitmay extend from ground by way of the contacts 248, 249, 232 and 22|, andthe winding of the relay R2I0 to battery, or alternatively from groundby way of the contacts 248, 249,233 and 222, and the winding of R2I0 tobattery, depending upon which of the two relays R230 and R220 isoperated. When energized over one of the two circuits just traced, therelay R2I0 operates after a short interval to complete a locking circuitfor itself, this circuit extending by way of the grounded privateconductor I Ic, the contacts 2| I, and the winding of R2I0 to battery.After this circuit is completed, the relay R2I0, at its contacts 2I2,opens one of the parallel locking circuits for the relay R240. In amanner similar to that just described, the other lockout relaysassociated withvthe line Il and corresponding to the other ringing codesare energized in response to nonconcurrent operation of the code andringing relay's by which they are respectively controlled. Each of theselockout relays, in operating, locks to the grounded private conductorIIc of the called line II. Since all of the lockout relays associatedwith the line II are held energized over the grounded private conductorIIc of the line II, it will be apparent that these relays are onlyreleased in response to the release of the switch train through whichthe call .is routed to the line II. In other words, the vlockout relaysare held operated for the duration of the call. With these relaysoperated the intercepting circuit 40 is positively prevented fromintercepting or otherwise interfering with the call which has beenrouted to one of the substations associated with the line II. Thus, withall of the lockout relays R2I0, R2I0', etc., energized, the primaryoperating circuit for the relay R240 is opened at the contacts 2I2,2I2', etc., so that this relay is fully released. When the connectioninvolving the line I I is subsequently released and the controlequipment operates to disconnect the private conductor I Ic from ground,the operated lockout relays are deenergized and restore. The callintercepting circuit 40 is thus fully restored to normal.

From the foregoing explanation it will be apparent that if ringingvoltage of the code 5 is impressed upon the positive line conductor IIato energize the ringing relay R230 and thus cause the partial operationof the line switching relay R240, the ringing relay R230 by concurrentlyoperating with the No. 5 code relay R200 will hold the operating circuitfor the code 5 lockout relay R2I0 open at the contacts 22|', 222', 232'and 233'. All of the other lockout relays associated with the line IIwill be energized in a manner clearly apparent from the precedingdescription. With the relay R2I0 deenergized, however, the primarycircuit for energizing the lower winding of the line switching relayR240 is not interrupted. Accordingly, at the end of the ensuing cycle ofoperation of the ringing interrupter, when ground potential is againimpressed upon the pickup lead C208, the line switching relay R240 willbe caused to complete its operation whereby the call is intercepted androuted to the operators position 45. In a similar manner, calls intendedfor substations associated with the line II and designated by ringingcurrents of the other three codes may be intercepted through theprovision of additional lockout relays.

If desired, each` of the lines arranged for call intercepting servicemay be terminated in a line circuit individual to the line and providedat the intercepting operators position 45, thus permitting the linesoccupied with intercepted calls to be identified by the operator. Thismode of arranging the circuit is indicated'by the dashline trunk 46a,the conductors of which extend from the contacts 24| and 242 of the lineswitching relay R240 directly to a line jack provided at the operatorsposition 45. In this case, the finder switch 210 and the circuits forcontrolling this switch may be dispensed with. More specically, if thefinder switch 210 is omitted and a straight connection.46a to theintercepting operators position 45 is used, the contacts 246 and 241embodied in the relay R240 may be omitted from this relay. Otherwise,the circuit arrangement will be exactly as disclosed.

From the foregoing explanation it will be understood that theintercepting circuit 40 is so arranged that operation of this circuitinadvertently to intercept calls routed to the lines served thereby, ispositively prevented. This is true for the reason that the ringingvoltage impressed on a line arranged for intercepting service is, duringthe first code cycle of each voltage, utilized for test purposes. If thecharacter of the ringing voltage impressed on the line' during thisiirst ringing period indicates that the call is not to be intercepted,the call intercepting apparatus individual to that line is locked up andpositively prevented from operating to route a connection to theoperators position 45. On the otherhand, if the call is to beintercepted, the

. intercepting operation is performed in response to the pickup pulseapplied to the pickup lead C208 at the end of the nrst ringing cycle.This, of course, means that the interception of the call is only delayedfor a short time interval.

While there has been described what is at present. considered to be thepreferred embodiment of the invention, it will be understood thatvarious modications may be made-therein and it is contemplated to coverin the appended claims all such modificationsas fall within the truespirit and scope of the invention.

What is claimed is:

1. In a telephone system, a line, an operators position, cyclicallyoperating ringing apparatus for developing one or more pulses of ringingvoltage during each cycle of operation thereof, a control deviceoperative in accordance with said voltage pulses, means for routing acall to said line from another line of the system and for impressingsaid ringing voltage pulses upon said line, a control relayenergized'byringing voltage impressed upon said line, a line switchingrelay partially operative in response to operation of said controlrelay, means for causing said line switching relay to complete itsoperation in the event said control device and said control relay aresubstantially concurrently operated during one cycle of operation ofsaid apparatus, and means responsive to the complete operation of saidline switching relay for extending a connection from said line to saidoperators position.

2. In a telephone system, a line, an operators position, cyclicallyoperating ringing apparatus for developing one or more pulses of ringingvoltage during each cycle of operation thereof, a control deviceoperative in accordance with said voltage pulses, means for routing acall to said line from another line oi' the system and for impress- 75ing said ringing voltage pulses upon said line, control relay energizedby ringing voltage iinpressed upon said line, a line switching relay-partially operative in response to operation of said control relay,means for causing said line switching relay to complete its operation inthe event said control device and said control relay are substantiallyconcurrently operated during one cycle of operation of said'apparatus,means responsive to non-concurrent operation of said control relay andsaid control device during said one cycle of operation of said apparatusfor deenergizing said line switching'relay, and means responsive to thecomplete operation of said line switching relay for extending aconnection from said line to said operators position.

3. In a telephone system, a line including talking conductors and aprivate conductor, an operators position, Ycyclically operating ringingapparatus for developing one or more pulses of ringing voltageduring'each cycle of operation thereof, a control device operative inaccordance with said voltage pulses, an automatic switch having accessto said line and including means for impressing said ringing voltageupon said line, means including said switch for routing a call to saidline from another line of the system. a control relay energized byringing voltage impressed upon said line, a line switching relay, aprimary operating circuit for said line switching relay, said primaryoperating circuit including said private conductor and vbeing completedto cause the partial opera-v tion of said line switching relay inresponse to seizure of said line by said switch and the operation ofsaid control relay, a secondary operating circuit for said lineswitching relay, said secondary operating circuit being prepared inresponse to the partial operation of said line switching relay, meansfor completing said secondary operating circuit to cause said lineswitching relay to complete its operation in the event said controldevice and said control relay are substantially concurrently operatedduring one cycle oi operation of said apparatus. and vmeans responsiveto the complete operation of said line switching relay for extending aconnection from said line to said operators position.A

`fi. In a telephone system, a line including talking conductors and aprivate conductor, an operators position, cyclically operating ringingappzaratus for developing one or more pulses of ringing voltage duringeach cycle oit-operation thereof, a control device operative inaccordance with said voltage pulses, an automatic switch having accessto said line and including means for impressing said ringing voltageupon said 1me means including said switch for routing a call to saidline from anotherline of the system, a. control relay energized byringing voltage impressed upon said line, a line switching relay, aprimary operating circuit for said line switching relay, said primaryoperatingcircuit including said privateconductor and'being completed tocause the partial operation of said line switching relay in response toseizure oi' said line by said switch and the operation of said controlrelay, a secondary operating circuit for said line switchingrelay, saidsecondary operating circuit being prepared in response to the partialoperation of said line switching relay, means for completing saidsecondary operating circuit to ca'use said line switching relay tocomplete its operation in the event said control device and said controlrelay are substantiallyconcurrently operated during one cycle ofoperation of said apparatus, means responsive to non-concurrentoperation of said control relay and said control device during said onecycle of operation of said apparatus for deenergizing said lineswitching relay, and means responsive to the complete operation of saidline switching relay for extending a connection from said line to saidoperators position? 5. In a telephone system, a line including talkingconductors and a private conductor, an operators position, cyclicallyoperating ringing apparatus including a pickup lead, means forimpressing a. pickup pulse upon said lead at the beginning of each cycle'of operation of said apparatus and means for developing one or morepulses of ringing voltage during each cycle of operation of saidapparatus, a control device operative in accordance with said voltagepulses, an automatic switch having access to said line, means includingsaid switch for routing a call to said line from another line of thesystem, said switch including means operative in response to a iirstpickup pulse impressed upon said lead 'to impress said ringing voltageupon said line, a

control relay energized by ringing voltage impressed upon said line, aline switching relay, a. primary operating circuit for said lineswitching relay, said primary operating circuit including said privateconductor and being completed to cause the partial operation of saidline switching relay in response to seizure of said line by said switchand the operation of said control relay. a secondary operating circuitfor said line switching relay, said secondary operating circuitincluding said pickup lead and beingcompleted to cause said lineswitching relay to complete its operation in response to the partialoperation oi said line switching relay followed by the application of asecond pickup pulse to said lead only in the event said control deviceandsaid control vrelay are substantially concurrently operated duringthe cycle of operation of said apparatus defined by said first andsecond pickup pulses, and means 'responsive to the complete operation ofsaid line switching relay for extending a connection from said line tosaid operators position. 6. In a telephone system, a line includingtalking conductors and a private conductor, an operators position,cyclically operating ringing apparatus including a pickup lead, meansfor impressing a pickup pulse upon said lead at the beginning of eachcycle of operation of said apparatus and means for developing one ormore pulses of ringing voltage during each cycle of operation of saidapparatus, a control device operative in accordance with said voltagepulses, an automatic switch having access to said line, means includingsaid switch for routing a call to said line from another line of thesystem, said switch including means operative in response to a firstpickup pulse impressed upon said lead to 4impress said ringing voltageupon said line, a

control relay energized by ringing voltage impressed upon said line, aline switching relay, a primary operating circuit for said lineswitching relay, saidprirnary operating circuit including said privateconductor and being completed to cause the partial operation of saidline switching relay in response to seizure of said line by said switchand the operation of said control relay, a secondary operating circuitfor said line switching relay, said secondary loperating circuitincluding said pickup lead and being completed to operation in responseto the partial operation of said line switching relay followed bytheapplication of a second pickup pulse to said lead only in the event saidcontrol device and said control relayare substantially concurrentlyoperated during the cycle 4of operation of said apparatus defined bysaid iirst and second pickup pulses,

' line to said operators position.

7. In a telephone system, a line adapted to have a plurality ofsubstations associated therewith, cyclically operating ringing apparatusincluding means for developing a plurality of differently coded ringingvoltages during each cycle of operation thereof, control relaysindividually corresponding to the ringing codes and each operativeduring each ringing interval of the correcause said line switching relayto complete its sponding code, means for routing a call to said linefrom another line of the system and for impressing ringing voltageI of-a selected code upon said line, and means jointly controlled by theapplication of ringing voltage to said line and the operation of apredetermined one of said relays for intercepting said call only in theevent the operation of said one relay is concurrent with the applicationof ringing voltage to said line for one complete cycle of operation ofsaid ringingapparatus.

. 8. In a telephone system, -a line adapted to have a plurality ofsubstations associated therewith, cyclically operating ringing apparatusincluding means for developing a plurality of differently coded ringingvoltages during each cycle of operation thereof,control-relaysindividually corresponding'to the ringing codes and eachoperative during each ringing interval of the corresponding code, meansfor routing a call to said line from another line of the system and forimpressing ringing voltage of a selected code upon said line, meansjointly controlled by the application of ringing voltage to said lineand the operation of a predetermined one of said relays for interceptingsaid call only in the event the operation of said one relay isconcurrent with the 'application of ringing voltage to said line for onecomplete cycle of operation of said ringing apparatus, and meansoperative in response to operation of said one relay at a time whenringing voltage is not impressed upon said line or to the application ofringing voltage to said line at a time when said one relay is notoperated for preventingthe operation of said last-named means.

9. Ina telephone system, a line adapted to y have a plurality ofsubstations associated therewith, cyclically operating ringing apparatusincluding a pickup lead, means for impressing a pickup pulse upon saidlead at the beginning of each cycle of operation of said apparatus andmeans for developing differentlycoded ringing voltages during each cycleof operation of said apparatus, control relays individuallycorresponding to said codes and each operative during each ringinginterval of the corresponding code, line switching apparatus for routinga call to said line from another 'line of the system and including meansoperative in response to a first pickup pulse applied to said lead forimpressing ringing voltage of a selected code upon said "line, and meansjointly controlled by the applicationof rinsing voltage to said line,the operation oi a predetermined one of said relays, and the applicationof a second pickup pulse to said lead for intercepting said call only inthe event the operation of said one relay is concurrent with theapplication of ringing voltage to said line during the cycle ofoperation of said apparatus defined by said nrst and second pickuppulses.

10. In a telephone system, a line adapted to have a plurality ofsubstations associated therewith, cyclically operating ringing apparatusincluding a pickup lead, means for impressing a pickup pulse upon saidlead at the beginning of each cycle of operation of said apparatus andmeans for developing differently coded ringing voltages during eachcycle of operation of said apparatus, control. relays individuallycorresponding to said codes and each operative during each ringinginterval of the corresponding code, line switching apparatus for routinga call to said line from another line of the system and including meansoperative in response to a first pickup pulse applied toA said lead forimpressing ringing voltage of a selected code upon said line, meansjointly. controlled by the application of ringing voltage to said line,the operation of a predetermined one of said relays, and the applicationof a second pickup pulse to said lead for intercepting said call only inthe event the operation of said one relay is concurrent .with theapplication of ringing voltage to said line during the cycle ofoperation of said apparatus deiined by said ilrst and second pickuppulses, and means operative in response to operation of said one relayat a time when ringing voltage is not impressed upon said line or to theapplication of ringing voltage to said line at a time when said onerelay is not operated for preventing the operation of said last-namedmeans.

1l. In a telephone system, a multiparty line, cyclically operatingringing apparatus including a pickup lead, means for impressing a pickuppulse upon said lead at the beginning of each cycle of operation of saidapparatus and means for developing differently coded ringing voltagesduring each cycle of operation of said apparatus,

line switching apparatus for routing a call to said y line from anotherline of the system and including means operative in response to apick-up pulse applied to said lead for impressing ringing voltage of aselected code upon said line, a code marking path individual to one ofsaid codes and to which ringing voltage oi said one code is applied bysaid ringing apparatus, and means operative in response to theapplication of a succeeding pickup pulse to said pickup lead forintercepting said call only in the event the code of the lringingvoltage applied to said multiparty line matches the code of the ringingvoltage applied to said marking path.

12. In a. telephone system, a multiparty line, cyclically operating'ringing apparatus including a pickup lead, means for impressing a pickuppulse upon said lead at the beginning of each cyclev of operation ofsaid apparatus and means for developing diiIerently coded ringingvoltages during each cycle of operation of said apparatus, lineswitching apparatus for routing a call to said line from another line ofthe system and including means operative in response to a pickup pulseapplied to said lead for impressing ringing voltage of a selected codeupon said line, a code marking path individual to one of said codes,

means-including at least a portion of said'ringingA apparatus fortransmitting signals oi' said one code over said marking path, and meansoperative in response to the ,application of a succeeding pickup pulseto 4said pickup lead for intercepting said call only in the event thecode of the ringing voltage applied to ssldmmupmy line matches the codeo! the signals transmitted vover 'said marking path.

13. In a telephone system, a multiparty line. cyclically operatingringing apparatus including a pickup lead, means for impressing a pickuppulse upcnsaid lead at the Vbeginning of each cycle of operation of saidapparatus'and means for developing diiierently coded ringing voltagesduring each cycle of operation of said apparatus. line switchingapparatus for routing a call to said line from another line of'thesystem and including means operative in response to a pickup pulseapplied to said lead for impressing ringing voltage of a selected codeupon said line. a code marking path individual to one of said codes,means including at least a portion of said rinsing apparatus fortransmitting signals of said one code over said marking path. and meansoperative in response to the application of a second pickup pulse tosaid pickup lead for intercepting said call only in the event the codesegments of the ringing voltage applied to said multiparty line matchtne code segments of the signals transmitted over said marking pathduring the cycle of operation o! said ringing apparatus which is dennedby saidilrst and second pickup pulses.-

14. In a telephone system, a multiparty line, an automatic switch havingaccess to said line through its outlet contacts', cyclically operatingringing apparatus'including a. pickup lead, means for impressing apickup pulse upon said lead at the beginning of each cycle of operationof said apparatus and means for developing diiierently codedringingvoltages during each cycle of operation ot said apparatus, meansincluding said switch for routing a call to said line from another lineof the system, said switch including means operative in response to theapplication of a pickup pulse to said leadlfor impressing ringingvoltage of a selected code on said multiparty line, a code marking pathindividual to one of said codes, means including at least a portion ofsaid ringing apparatus for transmitting signals of said one code oversaid marking path, and means jointly controlled over the outlet contactsof said switch-and over said marking path and operative in response tothe'application of a succeeding pickup pulse to said pickup lead forintercepting saidcall onlyiin the event the code of the ringing voltageapplied to V said multiparty line' matches the code of tne signalstransmitted over said marking path.

15. In a telephone system, a multiparty line, l an automatic switchhaving access to said lineV through its outlet contacts, cyclicallyoperating ringing apparatus including a pickup lead, means forimpressing a pickup pulse upon said lead at the beginning of each cycleof operation of said apparatus and means for developing dinerently codedringing voltages during each cycle party line, a code marking pathindividual to une` of said codes, means including at least a. portion ofsaid ringing apparatus for transmitting signals of said one code oversaid marking path, means f operative in response to the application of asucceeding pickup pulse to said pickup lead for intercepting said call,and means controlled jointly over said marking path and througn theoutlet contacts of said switch for preventing said call from beinginterceptedin the event the code of the ringing voltage applied to saidmultiparty line fails to match the code of the signals p 5 transmittedover said marking path.

WILLIAM H. RYAN.

